For general rape, the victim is typically capable of giving consent but chooses not to, meaning we know the rapist is violating them. For situations where the victim is incapable of consenting, it is true that we are assuming a position for them. As a society, we have observed that being made to have sex in a vulnerable position is a negative experience, so it makes sense to extrapolate they would be opposed if they were capable of choosing.

For life, the observation is different. Once people have the power to knowingly "opt out" of existing, they rarely do. Most people instead prefer existing and consider it to be positive. So we should assume a hypothetical person would also choose to be born when acting on their behalf.